Both companies admitted guilt and apologized to owners for the errors, promising to reimburse the 900,000 customers who had bought affected vehicles.

However, Automotive News reports that a lawsuit has been filed in U.S. District Court which seeks damages for U.S. customers who purchased or leased any of the 13 models involved.

While most models were found to be 1-2 mpg out on their true figures, some were awry by as much as 6 mpg.

Both Kia and Hyundai had set a compensation rate, based on how many miles each customer had driven, and how much money they would supposedly have lost out by the discrepancy. Owners would bring the vehicle into their local dealership to have the odometer checked, and this data would be used to calculate compensation.

As well as seeking damages and attorney fees, the lawsuit, if successful, will also allow customers to back out of purchase or loan agreements. The plaintiffs are 2012 Kia Rio owner Molly Simmons, and Jeffrey Millar, who recently bought a 2013 Hyundai Elantra.

Other models found to have inaccurate window stickers include the Hyundai Accent, Azera, Genesis, Santa Fe, Sonata Hybrid, Tucson and Veloster. Other Kia vehicles which require re-stickering include the Optima Hybrid, Sorento, Soul and Sportage.

Both companies have already amended window stickers to represent the true values.

Hyundai and Kia are reviewing the lawsuit, and neither is able to comment at this time.